PCB versions, features and guidelines
"Hacker Friendly" ZeroPhone PCBs
This is currently only version of PCBs - in the future, there will be likely many different options, and they are to be described here as they appear. This version of ZeroPhone PCBs is optimized for easy assembly by hand-soldering, as well as for the Chinese cheap PCB manufacturers' requirements. They also do not have any hard-to-get components, like ICs that you could only get on Digikey/Mouser/Farnell/etc., the components were chosen by how simple it was to get them on places like eBay, Aliexpress on Taobao.
For example, current ZeroPhone software is designed for use with a 1.3" 128x64 OLED screen, interfaced through SPI. Rather than adding an FPC footprint and all the capacitors and resistors (sometimes, even boost regulators) that are necessary for the panel, it's much simpler to add a 2.54 header for an OLED breakout board, and make it accept different kinds of displays while at it. It's going to be easier to solder it, since there's no need to solder an FPC with fine pitch and a handful of different passives, and a breakout is actually much easier to source for an unexperienced person, compared to sourcing a panel with an FPC. While this principle is not very compatible with mass manufacturing of ZeroPhones, it's a good choice of components for somebody that wants to assemble their own ZeroPhone independently and doesn't know much about sourcing parts from places that are typically more appropriate for sourcing.
Current "Hacker Friendly" PCBs are designed for "Pi Zero sandwiched between boards" design. The GPIO connections are done using the 40-pin GPIO connector - the back board has a 2x20 2.54 male connector, on which the Pi Zero is soldered, leaving some place for the front board's 2x20 2.54 female connector. This is both easy to solder and still leaving some modularity - although the Pi Zero being soldered means it's hard to split it from the back board (desoldering needles could help with that, but this is unlikely to be easily done by a heat gun). The back-front board interconnects are also done using 2.54 headers - the ATMega programming connections and the GSM modem audio are done this way. Moreover, the expansion headers are also 2.54 - single-row female angled headers.
"Hacker Friendly" PCBs also have more modularity-related features.
Keypad board features:
- is easily splittable from the front board (the desoldering is easier, as pins are split in smaller groups)
- has all the pins on 2.54 grid, so a replacement keypad board could even be prototyped using some stripboard
- is compatible with multiple footprints of side buttons available from eBay/TaoBao, making sourcing easier
- has VCC, GND, I2C SDA and SCL available on a header, along with an extra GPIO - so that it's easy to develop your own keypad boards with extra features, using various I2C-* enabled keypad controllers that are available from different IC manufacturers
- has two pins for LED backlight, that are controlled by a transistor placed on front board (currently there's no backlight designed in ZeroPhone keypad board, as that can only be reasonably well done after a case with transparent buttons is designed).